Influenza is the leading cause of death from a infectious cause and the 8th overall cause of death in the United States. Several groups of investigators who conducted autopsies on patients who died during the recent H1N1 influenza A pandemic reported that death was most commonly attributable to a severe epithelial lung injury with pathologic features of alveolar epithelial cell apoptosis and diffuse alveolar damage. Clinically, this is manifested as refractory acute hypoxemic respiratory failure and the acute respiratory distress syndrome (ARDS). Apoptosis, or programmed cell death occurs through activation of the intrinsic or extrinsic apoptotic pathway and is consistently observed in the lungs of animals and patients with influenza A infection. Whether activation of the intrinsic or extrinsic apoptotic pathway plays a clinically important role in the development of acute lung injury or the clearance of virus after influenza A infection is not known. In our preliminary data, we show the feasibility of using genetic mouse models to inhibit the extrinsic and intrinsic apoptotic pathway specifically in the mouse lung epithelium. We propose to test the hypothesis that activation of the intrinsic or extrinsic apoptotic pathway contributes to influenza A-induced cell death, lung injury, and viral clearance. We have formulated two interrelated specific aims to test this hypothesis: Aim 1: To determine whether activation of the extrinsic apoptotic pathway in the alveolar epithelium contributes to influenza A-induced lung injury. Mice deficient in FADD exclusively in the lung epithelium and mice globally deficient in BID will be subjected to influenza A infection for measurement of lung injury, inflammation and viral clearance. Primary alveolar epithelial cells from these mice will be used to examine the role of the extrinsic apoptotic pathway in influenza A-induced alveolar epithelial cell death. Aim 2: To determine whether activation of the intrinsic apoptotic pathway in the alveolar epithelium contributes to influenza A-induced lung injury. Mice deficient in BAX/BAK exclusively in the alveolar epithelium and mice globally deficient in BIM, PUMA and NOXA will be subjected to influenza A infection and analyzed as in Aim 1. This proposal combines lung epithelial specific knockdowns of apoptotic proteins with a clinically relevant model of lung injury and inflammation to elucidate important biologic mechanisms. This innovative approach will allow us to determine the pathophysiological role of alveolar epithelial apoptosis in influenza A-induced lung injury. These findings have the potential to uncover novel therapeutic strategies for the treatment or prevention of severe influenza A infection. PUBLIC HEALTH RELEVANCE: Influenza A infection is the leading cause of death from an infectious cause and is known to cause severe epithelial lung injury with evidence of apoptosis, or programmed cell death. This proposal seeks to determine the role of epithelial cell apoptosis in the development of influenza A-induced lung injury. These studies may identify novel targets for therapeutic drug development in influenza A infected patients.